Wind speed can be measured using a conventional three-cup rotational anemometer. Each cup of the anemometer is supported on a respective arm. The arms with attached cups are perpendicularly mounted to a rotational hub or shaft forming a rotational assembly that spins about the rotational axis of the anemometer. The rotational assembly is supported by a bearing attached to a support member such as a shaft. Each cup of the conventional anemometer is cone-shaped or hemispherical and rotationally symmetric about a central axis of the cone or hemisphere. Each cup has an opening or mouth which directs incident wind into the concave body of the cup when the cup is facing in a windward direction i.e. facing towards the direction from which the wind is blowing, and the cup is a “propelled” cup. When the cup is facing in a leeward direction, i.e. facing away from the direction from which the wind is blowing or facing towards the direction towards which the wind is blowing, the rounded or pointed back side of the cup deflects wind, and the cup is a “returning” cup. The difference between the forces on a windward facing “propelled” cup and a leeward facing “returning” cup propels the cups around the rotational axis of the anemometer.
The rotational axis of the anemometer is generally mounted or held as vertical or otherwise perpendicular to the wind flow, for a generally horizontal wind flow, with other mounting angles possibly applicable for other wind angles. Wind speed is related to the rotational speed of the cups about the rotational axis. As a result of symmetry of the cups and arms with respect to the rotational axis, accuracy of the anemometer is generally unaffected by wind direction, e.g. relative to a compass direction, as long as the wind flow is normal to the rotational axis of the anemometer and, equivalently, parallel to the plane of rotation of the rotational assembly. Errors are introduced with high-speed winds or with wind flow deviating from a normal to the rotational axis of the anemometer or equivalently wind flow arriving at an angle deviating from the plane of rotation of the cups. Known variations of the conventional three-cup rotational anemometer include differing numbers of cups and multiple groupings of the cups.
U.S. Pat. No. 3,541,855 to Frenzen et al. discloses an anemometer having two groups of arms and cups, which reduces an overspeeding error as compared to a conventional three-cup anemometer. Each group has three arms and three cups, and the two groups are mounted in spaced apart parallel arrangement perpendicular to the rotational shaft of the anemometer. An overspeeding error of the conventional three-cup anemometer, from wind deviating by an angle from the horizontal, results from wind flow passing over the top or under the bottom of an interfering cup to partially relieve the blocking effect thereof.
U.S. Pat. No. 6,895,812 to Dahlberg discloses an anemometer having at least two cups, with each cup having a substantially triangular shape and arcuate cuts. The corners of the triangles are rounded. An improvement in the flatness of the angular response of the anemometer is observed in winds having an angle of incidence deviating from a normal to the rotational axis of the anemometer. A horizontal sheet at the bottom of the cup, positioned in or parallel with the plane of rotation, reduces the driving moment of the cup and the overspeed of the anemometer for large negative or positive angles of incidence of wind.
EP 1398637 to Schmoling et al. discloses a rotational anemometer having an inclination of cone axes relative to the plane of rotation amounting to several tenths of a degree to several degrees. Inclination of the cone-shaped shells achieves an oblique incident flow characteristic that offsets the flow characteristic of a standard anemometer to correspond to an incident wind flow that is oblique from above the plane of rotation.
For an ideal anemometer, the true speed of the wind arriving at an angle of incidence relative to the plane of rotation of the cups of the anemometer would be measured by the anemometer as proportional to the cosine of the angle of wind incidence. It is a goal of the present invention to improve upon the accuracy of the conventional anemometer at various angles of wind incidence.